1. The Field of the Invention
The present invention is directed to processes for reducing nitrogen oxide emissions in combustion systems. More specifically, the present invention provides methods of decreasing the concentration of nitrogen oxides in flue gases emitted to the atmosphere from stationary combustion systems such as boilers, furnaces and incinerators.
2. The Relevant Technology
Nitrogen oxides are the major air pollutants emitted by boilers, furnaces, engines, incinerators, and other combustion sources. Nitrogen oxides include nitric oxide (NO), nitrogen dioxide (NO2), and nitrous oxide (N2O). Total NO+NO2 concentration is usually referred to as NOx. Combustion sources produce nitrogen oxides mainly in the form of NO. Some NO2 and N2O are also formed, but their concentrations are typically less than 5% of the NO concentration, which is generally in the range of about 200-1000 ppm. Nitrogen oxides are the subject of growing concern because they are toxic compounds, and are precursors to acid rain and photochemical smog. Nitrous oxide also contributes to the greenhouse effect.
Combustion modifications such as low NOx burners (LNB) and overfire air (OFA) injection provide only modest NOx control, reducing NOx concentrations by about 30-50%. However, their capital costs are low and, since no reagents are required, their operating costs are near zero. For deeper NOx control, Selective Catalytic Reduction (SCR), reburning, Advanced Reburning (AR), or Selective Non-Catalytic Reduction (SNCR) can be used in conjunction with low NOx burners and overfire air injection, or they can be installed as stand-alone systems.
Currently, SCR is the commercial technology with the highest NOx control efficiency. With SCR, NOx is reduced by reactions with nitrogenous reducing agents (N-agents) such as ammonia, urea, etc., on the surface of a catalyst. The SCR systems are typically positioned at a temperature of about 700xc2x0 F. in the exhaust stream. Although SCR can relatively easily achieve 80% NOx reduction, it is far from an ideal solution for NOx control. The size of the catalyst bed required to achieve effective NOx reduction is quite large, and use of this large catalyst, with its related installation and system modification requirements, is expensive to implement. In addition, catalyst deactivation, due to a number of mechanisms, typically limits catalyst life to about four years for coal-fired applications. The spent catalysts are toxic and pose disposal problems.
The reduction of NOx can proceed without a catalyst at a higher temperature. This is the SNCR process. It is effective over a narrow range of temperatures, or xe2x80x9ctemperature windowxe2x80x9d centered at about 1800xc2x0 F. where the N-agent forms NHi radicals which react with NO. Under ideal laboratory conditions, deep NOx control can be achieved; however, in practical, full-scale installations, the non-uniformity of the temperature profile, difficulties of mixing the N-agent across the full combustor cross section, limited residence time for reactions, and ammonia slip (unreacted N-agent) limit SNCR""s effectiveness. Typically, NOx control via SNCR is limited to 40-50%. Thus, while SNCR does not require a catalyst and hence has a low capital cost compared to SCR, it does not provide high efficiency NOx control. The most common SNCR N-agents are ammonia and urea, and the corresponding methods are called xe2x80x9cThermal DeNOxxe2x80x9d and xe2x80x9cNOxOUT.xe2x80x9d
The Thermal DeNOx process is described in detail in U.S. Pat. No. 3,900,554 to Lyon, and in Lyon and Hardy, xe2x80x9cDiscovery and Development of the Thermal DeNOx Process,xe2x80x9d Ind. Eng. Chem. Fundam., 25, 19 (1986). When ammonia is injected into combustion flue gas containing NO and oxygen at temperatures between about 1500 and 2000xc2x0 F., a series of chemical reactions occurs and NO is converted to molecular nitrogen. The reaction is believed to start with formation of NH2 radicals by reaction of ammonia with OH, O or H atoms:
NH3+OHxe2x86x92NH2+H2O
NH3+Oxe2x86x92NH2+OH
NH3+Hxe2x86x92NH2+H2
The main elementary reaction of the NO to N2 conversion is then:
NH2+NOxe2x86x92N2+H2O
Another SNCR additive is urea, (NH2)2CO, which is disclosed in U.S. Pat. No. 4,208,386 to Arand et al., and is used in the NOxOUT process. When added to combustion flue gases, urea is rapidly thermally decomposed to NH3 and HNCO:
(NH2)2COxe2x86x92NH3+HNCO
Thus, the mechanism of urea reduction of NOx includes the reactions of NH3 described above, as well as reaction of HNCO. The most important HNCO reactions with radicals are:
HNCO+Hxe2x86x92NH2+CO
and
HNCO+OHxe2x86x92NCO+H2O
As in the Thermal DeNOx process, NH2 radicals can either remove NO:
NH2+NOxe2x86x92N2+H2O
or form NO by reaction with HNO radicals. NCO radicals can remove NO to form N2O:
NCO+NOxe2x86x92N2O+CO
and then CO and N2O molecules are oxidized by OH and H, respectively:
CO+OHxe2x86x92CO2+H
N2+Hxe2x86x92N2+OH
Thus, the process has a similar narrow temperature window as NH3 injection, but can be complicated by N2O formation. The SNCR temperature window could be broadened to lower temperatures if an alternative source of OH radicals could be found. Attempts to do this have included addition of hydrogen or hydrogen peroxide to ammonia, alcohols to urea, etc. The action of most additives is to shift the temperature at which the de-NOx reactions are optimum, rather than to broaden the de-NOx temperature window. However, U.S. Pat. No. 5,270,025 to Ho et al. discloses several salt additives that considerably broaden the temperature window of the Thermal DeNOx process.
An alternative to controlling NOx emissions by SCR or SNCR processes is reburning. Reburning is a method of controlling NOx emissions via fuel staging. The main portion of the fuel (80-90%) is fired through conventional burners with a normal amount of air (about 10% excess) in a main combustion zone. The combustion process forms a definite amount of NOx. Then, in a second stage, the rest of the fuel (the reburning fuel) is added at temperatures of about 2000-2600xc2x0 F. into the secondary combustion zone, called the reburning zone, to maintain a fuel-rich environment. In this reducing atmosphere both NOx formation and NOx removal reactions occur. Experimental results indicate that in a specific range of conditions (equivalence ratio in the reburning zone, temperature and residence time in the reburning zone), the NOx concentrations can typically be reduced by about 50-70%. In a third stage, air is injected (overfire air, or OFA) to complete combustion of the fuel. Addition of the reburning fuel leads to the rapid oxidation of a portion of the fuel by oxygen to form CO and hydrogen.
The reburning fuel provides a fuel-rich mixture with certain concentrations of carbon containing radicals: CH3, CH2, CH, C, HCCO, etc. These active species can participate either in the formation of NO precursors in reactions with molecular nitrogen or in reactions with NO. Many elementary steps can share responsibility for NO reduction, and there is no commonly accepted opinion about their relative importance. The carbon containing radicals (CHi) formed in the reburning zone are capable of reducing NO concentrations by converting NO to various intermediate species with Cxe2x80x94N bonds. These species are reduced in reactions with different radicals into NHi species (NH2, NH, and N), which react with NO to form molecular nitrogen. Thus, NO can be removed by reactions with two types of species: CHi and NHi radicals. The OFA added in the last stage of the process oxidizes remaining CO, H2, HCN, and NH3 and unreacted fuel and fuel fragments. The reburning fuel can be coal, gas or other fuels.
The Advanced Reburning (AR) process is a synergistic integration of reburning and SNCR, and is disclosed in U.S. Pat. No. 5,139,755 to Seeker et al. In the AR process, an N-agent is injected along with the OFA, and the reburning system is adjusted to optimize NOx reduction by the N-agent. By adjusting the reburning fuel injection rate to achieve near stoichiometric conditions (instead of the fuel rich conditions normally used for reburning), the CO level is controlled and the temperature window for effective SNCR chemistry is considerably broadened. With AR, the NOx reduction achieved from the N-agent injection is increased. Furthermore, the widening of the temperature window provides flexibility in locating the injection system, and NOx control should be achievable over a broad boiler operating range.
The Advanced Reburning process provides an approach for increasing the OH concentration to form NH2 radicals from N-agents. It incorporates the chain branching reaction of CO oxidation into the process. When CO reacts in the presence of oxygen and water vapor (H2O), it creates free radicals including H, OH, O and HO2. Thus, if a controlled amount of CO from the reburning zone can be introduced at the point of N-Agent injection, the low temperature limitation of the window can be broadened and the NOx reduction enhanced.
Experimental studies have demonstrated two approaches for addition of OFA in reburning to prepare specific SNCR conditions. (Chen et al., xe2x80x9cAdvanced Non-Catalytic Post Combustion NOx Control,xe2x80x9d Environ. Progress, 10, 182 (1991)). One approach is to split the OFA addition and inject it in two stages so that the bulk of the oxidation is completed at the conventional OFA injection stage while a moderate amount of CO is left for burnout at a second injection stage at lower temperature where conditions are more favorable for DeNOx reactions. In an alternative approach, the reburning zone is deliberately de-tuned by increasing the stoichiometry to about 0.98-1.0.This allows a significant reduction in the reburning fuel flow, and eliminates one of the air injection stages. The basic AR process, i.e., CO-promoted N-Agent injection, shows that the temperature window can be broadened and NO removal efficiency increased if both CO and O2 concentrations are controlled to fairly low values (CO on the order of about 1000 ppm, and O2 at less than about 0.5 percent). At the point of air addition, CO and O2 are both low because of the close approach to SR=1.0.
U.S. Pat. No. 5,756,059 to Zamansky et al. discloses an improved Advanced Reburning process in which the N-agent can be injected under fuel rich conditions or at two injection locations, one each under fuel-rich and fuel-lean conditions, for deeper NOx control. The N-agent can be injected with or without promoters at one or two locations in the reburning zone, along with OFA or downstream in the burnout (SNCR) zone. The promoters are water-soluble inorganic salts that can be added to aqueous N-agents, or to solid, liquid or gaseous N-agents, and injected along with the N-agents to enhance the N-agent efficiency. In pilot scale AR experiments, NOx reduction of up to 95% was achieved. The estimated total cost of NOx control for AR systems is approximately half of that for SCR.
The chemistry of AR is no different than that for basic reburning and SNCR, and the reactions discussed above proceed. The critical difference is how the two sets of chemical reactions are synergistically integrated together. The final OFA initiates the oxidation of CO from the reburning zone:
CO+OHxe2x86x92CO2+H
H+O2xe2x86x92OH+O
O+H2Oxe2x86x92OH+OH
This chain branching sequence provides additional OH radicals to initiate the NH3 oxidation sequence:
NH3+OHxe2x86x92NH2+H2O
NH2+NOxe2x86x92N2+H2O
While prior systems are capable of controlling NOx emissions, even the most effective systems are still complex. In addition, effective NOx reduction systems can be expensive to implement, operate and maintain. Thus, there is a need for simpler, less expensive, and effective processes for reducing the NOx concentration in combustion flue gases.
It is an object of the present invention to provide methods for decreasing the concentration of NOx in combustion flue gases before the gases are emitted to the atmosphere.
It is another object of the present invention to provide relatively simple and inexpensive methods for decreasing the concentration of NOx in combustion flue gases.
It is another object of the present invention to improve the efficiency of NOx removal in conventional reburning processes.
These and other objects and advantages are achieved by providing methods for removal of nitrogen oxides from combustion flue gas wherein the combustion flue gas is contacted with certain metal-containing additives which advantageously and surprisingly reduce NOx alone or in conjunction with conventional NOx removal processes.
In one method according to the present invention, the concentration of nitrogen oxides in a combustion flue gas is decreased by providing a metal-containing additive in the main combustion zone. This method includes the steps of: providing a combustion zone for oxidizing a combustible fuel with an oxidizing agent, the combustion forming a combustion flue gas that contains nitrogen oxides; introducing a metal-containing additive in the combustion zone (separately or with fuel or air); and allowing the metal-containing additive to react within the combustion flue gas to decrease the concentration of nitrogen oxides therein.
In another method according to the present invention, the concentration of nitrogen oxides in a combustion flue gas is decreased by providing a metal additive in the reburning zone. This method includes the steps of: providing a combustion zone for oxidizing combustible fuel with an oxidizing agent, the combustion forming a combustion flue gas that contains nitrogen oxides; adding a reburning fuel to the combustion flue gas in a reburning zone; introducing a metal-containing additive in the reburning zone (separately or with the reburning fuel); and allowing the metal-containing additive to react within the combustion flue gas to decrease the concentration of nitrogen oxides therein. It has been surprisingly found that addition of a metal-containing additive in the reburning zone is effective to reduce NOx in the absence of N-agents. Optionally, an N-agent and/or overfire air can be added to the combustion flue gas downstream of the reburning zone to further increase NOx control.
In another method according to the present invention, the concentration of nitrogen oxides in a combustion flue gas is decreased by providing a metal additive in both the combustion and reburning zones. This method includes the steps of: providing a combustion zone for oxidizing a combustible fuel with an oxidizing agent, the combustion forming a combustion flue gas containing nitrogen oxides; introducing a first metal-containing additive in the combustion zone; allowing the first metal-containing additive to react within the combustion flue gas to decrease the concentration of nitrogen oxides therein; adding a reburning fuel to the combustion flue gas to form a reburning zone; introducing a second metal-containing additive in the reburning zone; and allowing the second metal-containing additive to react within the combustion flue gas to further decrease the concentration of nitrogen oxides therein. Both metal additives can be added separately or with combustion reagents.
These and other objects, features, and advantages of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.